Loss of power

DXZeff

New Member
Dec 12, 2019
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First a little context.
My engine ran great, once it was broken in it could push just north of 30MPH and could put hills behind it without much trouble. Unfortunately the exhaust began to leak around the gasket, this had happened before because when I got the engine the threads were stripped, this was actually the fourth time I'd had to tap new ones, the aluminum there was always noticeably dull and parts would crumble off in my fingers around the edges. Whilst going to tap more threads I decided to shine a light into the cylinder to see how things looked in there, the carbon build up didn't surprise me due to the conditions the engine operates in, I pull a trailer and it overheats if I don't run it a little rich and use slightly more oil, so that was no big deal. What did alarm me, however, were some large pits in the cylinder wall, though now I had an answer to some issues the engine always had, in that even when it was running well, it would occasionally backfire and then stall at idle.
The metal also seemed too weak to tap new threads for the exhaust again, so I decided to ditch that cylinder. The piston rings were very worn where they passed by those pits, along with dark stains that suggest blowby was happening there, overall I think getting rid of that cylinder was a good thing as the weak metal coupled with the pitting and blowby looks to me like a catastrophic failure waiting to happen. Also a defect at the bottom showed that it had been blowing out by one of the studs, explaining a small oil/gas leak I could never quite pinpoint.

Now the problems begin. I had a spare cylinder, so I ordered new gaskets, new rings, cleaned everything up and fitted the new cylinder, it may be worth noting that I'm comfortable with things like this as my Dad was a diesel engineer and I've spent a lot of time in the past working on engines with him, whilst I admittedly have much less experience with gas engines, 2-strokes are extremely simple and so don't really intimidate me, I'm always willing to learn things I don't know yet. Needless to say the installation went well and the engine started first time, in fact better than it ever had before.
At this point it all seemed good, there was noticeably more compression than before and it ran smoother, it also ran hotter but that didn't really concern me because the old one had when it was new, seemed logical given the new parts, now the tedium set in as the engine would surely have to be broken in again.
After clocking up good miles on shorter, slower trips I decided to open up the throttle but the engine didn't want to top 18MPH. This was slower than it was at this stage on the old cylinder, but I didn't worry as it was running smooth and break in wasn't complete yet, figured it would come good once that was achieved. Plus it seemed to be running slightly richer than before and as that wasn't about to do anything too concerning it seemed reasonable to leave tweaking until break in was done.

Several tanks of gas later and with good mileage on the clock the engine won't top 25MPH and takes a long time to get there. If a headwind hits or there's a decent gradient it just won't push through it and can drop to 15MPH.
I've played with the carb, cleaning it and adjusting the needle to all four settings, though it did about what I expected, made the engine run too rich or too lean, the second notch up proving to be the best and this made sense to me as I'm at a low altitude, actually almost 15' below sea level in places here (and yeah, we flood), so more oxygen at this altitude means things will go lean way faster than if I lived somewhere at higher elevation, or I think so anyway, by all means correct me if this is wrong.
Cleaned out the muffler (stock) somewhere around here but it made no difference. New gasket to be sure.
Tried with slightly less oil in my fuel or a short while (I usually never move far from 16:1 due to afore mentioned load causing lots of heat) but no observable difference except slightly less smoke, maybe 1MPH faster at best.

Checked for air leaks around the carb, found none but did find out that 15mm John Guest fittings have O-rings in them that someone might find useful for sealing their carb to the intake. Notably where that manifold meets the engine it is smaller on this cylinder, but the tube and inlet port are the same diameter, the inside of the tube is roughed up but the port is smooth on the inside, also the transfer ports in the cylinder are smoother on this one than the old one, which should be better, surely?
Checked for compression leaks, found none.
Cleaned air filter (stock) but no change.
Replaced spark plug, no difference, plug goes brown (milk chocolate color) after some use, a little oily though that's to be expected.
Replaced the exhaust with an upgraded one, sounds good but performs worse. If I make the carb richer the engine hits 21MPH dead on and then starts sounding like it's 4-stroking as it loses speed for a few seconds before picking up again. Making the carb leaner it has less power and gets hot fast, to the point it smells really bad, makes a slight clattering noise and I shut it off. Notably it does take off faster with this pipe when the fuel mix is right though. This is the one with flex pipe, my frame is fat and most don't fit by the downtube.
Cylinder walls are nice, shiny and smooth by this point.
Even checked the bearings in my back wheel as it squeaks if I push the bike backwards, but it seems to be the gear cassette, I'm good at ruining those as I have fairly strong legs and usually strip teeth within a week on any modern bicycle before the pawls let go - I can actually pedal the bike to around 35 without the engine and never downshift, derailleur gears seem too fragile to handle this well, probably comes from years of riding heavy things with only 1-3 gears.

This is as far as I have gotten, sorry it's such a huge wall of text but I wanted to be thorough. I searched around and found many threads on these boards, but none that seem to fix my problem. I don't care too much about the speed, per se, as I can't do more than 10MPH with the trailer on (it starts bouncing) and otherwise never really thought the engine seemed to like going far over 20MPH anyway, but I know it should go faster, so it irks me a little and I want the power back, because getting up the hill with the trailer on is hard work now as I often have to help it considerably.

Something is obviously wrong and I'd like to get to the bottom of it if possible, figured I may have missed something, maybe something very simple knowing my luck. Worst case scenario I suppose I could just ditch the whole engine for a new one, but as nothing seems to be badly wrong with it I'd sooner try to find the problem and fix it.

Thanks for any help with this, even if you've got nothing, I at least appreciate you taking the time to make it through the novel's worth of words up there.

EDIT:
The thought struck me that I could tape my helmet camera to the frame as some people might work better with sight and sound (i.e. me) but be warned, it's loud. Also forgot to mention, this is back on the stock exhaust, also this was running on the slightly leaner side than usual.
 
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ZekeNoScooter

New Member
Jun 18, 2019
19
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I have a similar problem, no top end, although I too out higher than you. Mine feels like it's got more to give just can't breathe. If I take my air cleaner off, I get probably 20% more at the top. Right on past 30mph. I just got a jet kit, and I'm making my own air cleaner.
But your problem is opposite but with same result. Air cleaner off and runs worse. It sounds like probs all began with swapping in that cylinder. Maybe go to that point and start again? Maybe the whole top end, piston and all? So that no mismatch issues? Best I got man. I'll follow the thread to see how you did.
GL and ride safe.
 

DXZeff

New Member
Dec 12, 2019
11
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Indeed, seems like you should be on track for an improvement with what you're doing.

After thinking about it some more, I think the concept has at least proven itself. As the bike and engine kit were so cheap, I'll keep running it for another month or so and then simply replace the whole lot with a new one, slightly better bike, slightly better engine. By the time I get to the bottom of any issues with this one by now I may as well have just bought a new one and really, that's probably the cause, that I didn't expect it to still be around this far into the future and the parts are just really cheap, this was a sub-£70 kit.

What I probably will do, however, is dismantle the engine after the replacement is up and running, then post back here if I find anything strange going on in there, in case it proves useful for someone else down the line. In the mean time I drilled out my baffles slightly which has at least given me just a tiny bit more power at lower RPM for the hill, or so it seems.

Thanks for the input though.
 

ZekeNoScooter

New Member
Jun 18, 2019
19
7
3
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Hey, here s a suggestion. My first kit was a super cheap 50cc. Crash and burn. Next swing was a local "expert" had a running 80 on a broken bike. Sold to me and even installed with all my old kit or parts for only $100 (engine only) that failed. Then I stopped. Nearly gave up. This time, I bought a quality engine only, reputable seller, paid a little more. And I waited. Got rid of the cheap kit parts (carb, exhaust, CDI) and I learned. I learned a ton here. More from some vendor's. More watching YouTube. Learned most important things: no 2 of these beasts are the same. Ask 5 people, you'll get at least at 3 answers. And don't be afraid to spend a little $ and even then expect it to get broken in shipping.
Oh, and my favorite: "I probably screwed it up myself". Try starting your motor with CDI wires connected backward. Won't start, advance is wrong. But it will, accidentally fire up with the motor turning backwards. That was 10 days troubleshooting, to figure it out. It's funny now...
 

Goblin

New Member
Dec 29, 2019
4
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Hey everyone im kind of new on here and i wanted to see if i could get some troubleshooting help on my new 2 stroke build.

I have a flying horse 2 stroke engine that i am breaking in and I am running a 50:1 mix ratio. The engine and everything are brand new and haven't been fully broke in yet. Also, my carb is an NT carb. My issue is that I feel like I am loosing power when i ride and i cant tell if its because the engine isn't worn in yet or because its another issue. I suspect another issue. The Bike starts well and runs fine but lacks power (only 10 mph or less) and it struggles on even small hills. I have adjusted my idling screw and needle clip (tried and tested every groove) and have drained my tank and added new fuel. Based on my plug chops and testing to see if the spark plug is oily or dry then it looks like my mix is fine so I am kind of stumped on what to do from here. I had a previous engine that was on a build i bought and it was really fast and ran great like it should have so i am wondering if it was because it was already broken in or if it could be something else like the jet. No idea what jet is in the new engine. i was thinking of possibly cannibalizing the other jet off the old motor since its busted ( mounting studs broke and block broke when i tried to dig them out. I am pretty mechanically inclined so I am willing to experiment a little. Everything on my new engine is stock.